The major objective of the Midwinter Conference of Immunologists is to provide an annual scientific meeting conducive to interaction among established senior scientists with new investigators, postdoctoral and graduate students. The goal of the conference is to communicate the most recent developments in the field of immunology. The Midwinter Conference of Immunologists has a long track record of organizing successful meetings. Immune System Development and Function is the general theme of the 43rd Midwinter Conference of Immunologists scheduled for January 2004. Important developments in understanding how the immune system is turned on and off will be presented in two separate sessions. New insights in the role of the innate immune system in the recognition and control of the immune responses to microbes will be explored. Host defense and biodefense and ways to analyze and augment responses to infectious agents will be addressed. Novel information on the identification and analysis of autoimmune disorders will be discussed. The speakers selected for the 43rd Midwinter Conference are excellent and represent well-known scientists as well as upcoming young investigators. The program will allow ample time for discussion, exchange of ideas and establishment of collaborations. The 2004 conference promises to be as successful as previous Midwinter Conferences to generate novel ways to explore the immune system.